466 EMBRYOLOGY OF THE TURTLE. Part III. 
in quite large cells; in fact, an egg little more than one sixteenth of an inch 
in mean diameter (PI. 8, fig. 21) contains numerous cells of considerable size, 
(Pl. 8, fig. 21a,) no one of which contains a mesoblast. Nor can it by any pos- 
sibility be advocated, that these cells are the contents of other cells, for no 
others exist; even in a much larger egg, up to full-grown ones, this holds 
good just as undoubtedly, for in such a mass of yolk as larger eggs contain, the 
mesoblasts and ectoblasts have respectively very peculiar and unmistakable prop- 
erties, not to be confounded with any other cell contents. The resemblance 
which this mode of cell formation bears to that commonly received is far more 
apparent than real; yet, paradoxical as it may seem, we must confess that it 
is very difficult to express the essential character of the difference which sepa- 
rates these two different modes of viewing the subject. 
An ectoblast, which, under a power of five hundred diameters, appears about 
one-eighth of an inch through, in its greatest diameter, (Pl. 8, fig. 21a,) has 
attained its greatest transparency. The ectoblasts preserve this remarkable trans- 
parency up to the full accomplishment of their growth, which is not reached, 
however, till the egg is fully ripened. 
The ectoblast has a remarkable plasticity and resilience, which the mesoblast 
also shares, existing from the youngest (PI. 9, fig. 8a, K) to the oldest stages, (PI. 
9, fig. 11h,) im consequence of which the cells may squeeze and worm their way 
among each other, and yet, when free, return to their origimal rotundity. Nor is 
this all; for, besides their impressibility, they have an equally great extensibility, 
(Pl. 9, fig. 8a, A, M, N,) which may best be seen after they have been left in 
contact for a while and then forced apart, by setting the ambient fluid in motion, 
when their adherent portions stretch out with long arms. However, beyond a 
certain extension of these projections the agglutination ceases, and each cell returns 
to its proper sphericity. 
Beyond the fact that the slightly irregular rough surface of the smaller cells 
becomes, in the larger cells, a remarkably smooth, polished, yet not glittermg 
superficies, presenting on the whole the appearance of a beautiful sphere of glass, 
we have nothing further to add as regards this special part of their organization, 
except to mention the action of heat, which collapses the cell wall upon its 
mesoblast, the latter remaining undisturbed, whilst the entoblasts are fused into a 
1 Properly to consider this subject, it should be time of its birth, we may now complete the descrip- 
introduced when investigating the mode of origin tion of its phases in a few words, and then proceed 
of the mesoblast, as we view this process. Since to describe the development of the primary altera- 
the growing ectoblast, apart from its mesoblast and tion of its homogeneity, that is, the initiative step in 
entoblast, varies but little, excepting in size, from the the formation of the mesoblast. 
